This application is requesting 5 years of funding to identify genes that play a major role in a multifactorial mouse model of seizure susceptibility involving DBA/2 (D2) and C57BL/6 (B6) mice. Over the past 5 years, we used quantitative trait locus (QTL) mapping to characterize the polygenic nature of this model and document the presence of a major genetic effect, termed Szs1, originating from the distal portion of chromosome 1. Then, using congenic strains, we narrowed the region that harbors Szs1 to a defined segment of DNA (3-6 cM) with a strategy in place that will allow additional significant reductions in the size of this critical interval. Thus, the prime focus of this application is to identify the gene, which underlies Szs1. Specific Aims of the current proposal include: 1) continuation of congenic strain development for reduction of Szs1 critical interval to 1-2 cM and for study of Szs1 modifying loci; 2) molecular analysis and strain sequence comparison of a select group of candidate genes from the critical interval; 3) functional studies of the significance of a threonine/serine polymorphism in a potassium ion channel candidate gene KCNJ10 including in vitro (electrophysiology) and in vivo (transgenesis) experiments; 4) construction of a physical map of the Szs1 critical interval. The first 3 Aims will be pursued in parallel fashion with continued, systematic refinement of the Szs1 critical interval directing a search for the underlying seizure susceptibility gene. Functional KCNJ10 studies will be undertaken immediately and could redirect planned strategy if results provide evidence that this K+-channel gene represents Szs1. If analysis of known gene candidates does not result in identification of Szs1, a physical map will be constructed and used to discover novel genes, which can then be studied as Szs1 candidates. Overall, this set of Aims represents a logical extension of 5 years of previous work and it is believed that the proposed congenic strain-candidate gene-positional cloning strategy will lead to the identification of a fundamentally important seizure susceptibility gene in mice. Hopefully, these findings will lead as well to new and important information regarding the pathogenesis of human epilepsy.